This invention relates to a method for determining carboxylic acid and a system for carrying out the method.
At present, it has been attempted to improve and control the quality of various food products containing carboxylic acid based on the results of determination of the amount of the carboxylic acid contained in the materials or semiprocessed foods present in the processing stages of the production of the food products or determine the quality of the food products by detecting the amount of carboxylic acid contained in the final food products. In the field of clinical medicine, any abnormal condition or disease of a living body is diagnosed by detecting or determining the value of carboxylic acid within the living body and then comparing the detected value of carboxylic acid with a predetermined normal value of carboxylic acid. In this way, the carboxylic acid determination is considered important in various fields.
Determination of carboxylic acid has been conventionally conducted by (1) preparing an active derivative of carboxylic acid such as carboxylic acid chloride, carboxylic acid ester or carboxylic acid anhydride, for example, causing the derivative to react with hydroxylamine to form hydroxamic acid, causing the hydroxamic acid to react with triferric ion to form ferric hydroxamate and finally, directly observing the color development of the ferric hydroxamate or determining the absorptivity of the ferric hydroxamate in a known procedure, (2) by determining variations in the color development of carboxylic acid through the addition of a pH indicator to the carboxylic acid, (3) by causing carboxylic acid to react with an oxidization reagent and then determining the consumed amount of the reagent or the amount of the reaction product or (4) by determining variations in the pH of the carboxylic acid by a pH meter.
Out of the various prior art methods for determination of carboxylic acid referred to above, method (1) is the most commonly practiced method, but if water is present in the formation of an active derivative from carboxylic acid such as carboxylic acid chloride, carboxylic acid ester of carboxylic acid anhydride, the method will encounter obstacles. Therefore, this method has the disadvantage that a special expedient is incorporated therein such as the use of a strong dehydrating agent in the case of a water soluble carboxylic acid sample. Method (2) is a method in which the acidic property of carboxylic acid is determined, but this method also has the disadvantages that when an acidic substance such as inorganic acid or sulfonic acid coexists with carboxylic acid, the determination of the carboxylic acid is impeded to a degree that the determination becomes impracticable and the determination accuracy varies substantially depending upon the type of carboxylic acid employed. Method (3) is not a method which utilizes a peculiar reaction for carboxylic acid and the determination is greatly impeded when any substance which is easily subject to oxidization coexists with the carboxylic acid. Finally, method (4) has disadvantages substantially the same as those described in connection with method (2) hereinabove.
Therefore, the object of the present invention is to provide a method for determining carboxylic acid which can effectively eliminate the above-mentioned disadvantages inherent in the prior art determination methods for carboxylic acid.
A further object of the present invention is to provide a method for determining carboxylic acid in a simple and precise manner.
Another object of the present invention is to provide a method for determining carboxylic acid in which only carboxylic acid is selectively determined by directly forming hydroxamic acid from carboxylic acid and hydroxylammonium salt whether any substance which is easily subjected to oxidization coexists with the carboxylic acid or not.
As one aspect of the present invention, there is provided a method for determining carboxylic acid which comprises the steps of causing a sample of carboxylic acid to react with hydroxylammonium salt in the presence of carbodiimide to form hydroxamic acid and then causing the thus formed hydroxamic acid to react with a ferric compound to form ferric hydroxamate.